Revert "Change jobs option default to 1" #13981

Job	Run time
regression (macos-13, ocaml-variants.4.14.2+options,ocaml-option-flambda)	20m 35s
regression (ubuntu-22.04, ocaml-variants.4.14.2+options,ocaml-option-flambda)	7m 21s
extraction	0s
	27m 56s

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